Conventional hydrocarbon-based polymers generally lack mechanisms for bringing about crosslinking via low-temperature or room-temperature cure (e.g., moisture cure or addition-cure). Such cure pathways are, however, well known in the case of siloxane polymers, and the development of such facile cure mechanisms for the organic systems is highly desirable. Thus, for example, a telechelic organic polymer having suitably reactive end-groups could be reacted through chain extension and/or crosslinking schemes to produce cured compositions having precise molecular weight between crosslinks, and therefore, more predictable and controllable properties.
A small number of telechelic (i.e., having two identical reactive end-groups) hydrocarbon polymers are known and these are generally prepared by anionic or cationic polymerization of olefins. However, only a few telechelic hydrocarbon polymer systems having endgroup functionality approaching 2.0 are known and only a handful of these have reactive or potentially reactive silyl or siloxane groups at the ends (see, for example, Kennedy et al., J. Polym. Sci. Polym. Symp., V. 72, 73, 1985; Marmo et al., Macromolecules, V. 26, 2137, 1993; U.S. Pat. No. 4,316,973 to Kennedy; and European Patent Application 0520279). However, to the best of the inventor's knowledge, the synthesis of conjugated diene-based polymers having at least 70 mol percent 1,4 microstructure and containing a reactive silyl species at each end, wherein the molecular weight can be effectively controlled during synthesis, has not been disclosed.
A viable method for synthesizing telechelic polydiene having a silyl group at each end comprises anionic polymerization of a diene monomer using a difunctional metal-based initiator (i.e, one capable of simultaneously initiating two polymer chains which grow outward from the initiator site). Two types of such initiators are available. The first (initiator A) is based on alkali metals such as Li, Na, or K in conjunction with an aromatic compound such as naphthalene; this system is generally used in a polar solvent (e.g., tetrahydrofuran). The second (initiator B) is a dimetallated, usually aromatic, hydrocarbon; this initiator can be used in polar or non-polar reaction environments. Those skilled in the art will recognize that the reaction conditions during initiation and polymerization can have profound effects on the molecular weight, molecular weight distribution and structural characteristics of the resulting polymers, particularly in the case of polydienes. Thus, in polar solvents such as tetrahydrofuran, initiator A can provide telechelic polymers having narrow polydispersity for monoolefins such as styrene and for siloxanes. In non-polar solvents, however, initiation can be heterogeneous, resulting in a broad molecular weight distribution and poor control over molecular weight. This is due to the very poor solubility of the metal/naphthalene dianionic initiator in a non-polar solvent. On the other hand, initiators of type B have slightly higher solubility in non-polar solvents and are, therefore, better suited for use therein.
From a structural perspective, anionic polymerization of dienes such as butadiene and isoprene is extremely sensitive to the polarity of the solvent used in polymerization. Polar solvents such as tetrahydrofuran (THF), even at very low proportions in an otherwise non-polar environment, lead to a high degree of 1,2 addition with butadiene and 3,4-addition with isoprene. However, it is the 1,4-addition mode which is highly desired since this structure imparts a relatively low glass temperature and can be formulated to provide good elastomeric properties. For these monomers, a non-polar solvent and, therefore, initiators of type B are required to obtain useful elastomeric polymers. Thus, when a type A initiator, such as the lithium naphthalene catalyst of the examples in European Patent Application 0520279 is used to polymerize a diene in a non-polar solvent, a small amount of a polar solvent is needed to prepare and (presumably) solubilize the catalyst. This, in turn, leads to a relatively low 1,4 addition, as shown in the examples of this European patent application, and any rubbery properties of the resulting copolymers are probably derived from the siloxane component.
In view of the above stated problems associated with type A initiators, a number of dianionic initiators of type B have been developed and used to polymerize diene monomers. For example, 1,3-bis(1-lithio-1,3-dimethylpentyl)benzene initiator, the synthesis of which from m-diisopropenylbenzene is shown in equation (1), has been used in a non-polar reaction environment by Lutz et al. in Polymer, V. 23, 1953, 1982 to polymerize isoprene and styrene. ##STR1##
Similarly, a family of difunctional initiators based on a double 1,1-diphenylethylene structure is known to be an effective initiator for the synthesis of styrene-isoprene-styrene and styrene-butadiene-styrene triblock copolymers when used in conjunction with an amine or alkoxide polymerization adjuvant (see Tung et al., Macromolecules, 11, 616, 1978 as well as U.S. Pat. Nos. 4,960,842; 4,205,016; 4,196,154; and 4,182,818). For example, the preparation of this type of difunctional initiator, 1,3-phenylene-bis(3-methyl-1-phenyl-pentylidene)bis(lithium), from 1,3-bis(1-phenylethenyl)benzene is shown below in equation (2) below. ##STR2## In the above equations, Bu.sup.s denotes secondary butyl radical. However, when the above mentioned 1,3-phenylene-bis(3-methyl-1-phenyl-pentylidene)bis(lithium) initiator and amine polymerization adjuvant are used to polymerize, e.g., isoprene, capping of the resulting living polymer dianion with a chlorosilane or a cyclic siloxane is inefficient. For example, the fraction of active ends capped with the chlorosilane was less than about 70%. Such a low level of endcapping is considered unacceptable, e.g., in the formulation of a curable composition, wherein the end-groups are to serve as cross-linking sites. It is also known in the art that dianionically active initiators and polymers exhibit strong and complex association phenomena, particularly in non-polar media. For dianionically active polydienes, such as dianionically active isoprene, it is believed that the above mentioned association is responsible for the inefficient end capping.